That is a real bad incident and you can tell your friend that he has been fortunate that it was diesel in a petrol car rather vice-versa. Because petrol in a diesel engine can cause a lot of damage to basic structure and design of the engine. Causing a lot of pain in terms of finances and otherwise.
In this case, engine will die as soon as diesel is sucked into the cylinders as it will not ignite due to difference in compression ratings.
* Now what he should do ?
---> He should call up VW after sales support immediately and let them know of the problem. This should be done at the earliest without any delay.
* How much would be the damage on engine front ?
---> No idea how much will VW charge for the clean up. It will also depend on whether filter needs to be replaced, which is in most circumstances will have to.
* Can he take any action against Bunk people / owner ?
---> I am a little hazy on this. Not sure if you can legally take any action against the fuel owner as this is a human error from both ends. From your friend for not asking for petrol and from the fill guy to not check what fuel needs to go in.
Please see below for more technical details:- Google Answers: Accidentally filled a gas engine with Diesel...how much am I out?
EDIT: For other people reading this, in case of such a scenario, DO NOT START your or even attempt to do it once you know that wrong fuel has been filled. Just tow away the car and ask the helpline for support!